Power supply



` June 24, 1958 J.-A. TORRENCE POWER SUPPLY 2 Sheets-Sheet 1 Filed June14, 1957 .All uit Sm, SQ

llll lill w3 S June 24, 1958 J. A. 'roRRENcE 2,840,760

POWER SUPPLY 2 Sheets-Sheet 2 Filed June 14. 1957 United States Patent OPOWER SUPPLY James A. Torrence, Chicago, Ill., assignor to Motorola,Inc., Chicago, Ill., a corporation of linois Application June 14, 1957,Serial No. 665,752

Claims. (Cl. 315-104) This invention relates to controlled powersupplies for electronic equipment and more specifically to powersupplies for television receivers.

Television receivers of the type in general use and manufacture use anumber of thermionic vacuum tubes which range from to more than 20. itis necessary to supply each of these tubes with a source of power usedto heat the cathodes and also a source of direct current power which iscontrolled to produce the sound and picture signals. Fi`he tubesgenerally in use are of a type having indirectly heated cathodes, inwhich the cathode consists of' a sleeve having a coating which emitselectrons when the sleeve is heated. The heating element is a separateunit enclosed within the cathode sleeve. The heating element is composedof a wire filament generally having a high positive value temperaturecoefficient, which means that as the temperature of the filament andcathode rises the resistance of the cathode heating element increases.This positive temperature coeicient has the effect of regulating thetemperature of the cathode by controlling the current and hence thepower supplied to the cathode. At normal room temperature the resistanceof the heater element is very low so that when normal voltage is appliedto the vacuum tube heaters a very large initial surge of current occursmaking the thermal gradient in the region around the heater extremelyhigh and having a destructive effect on the heater wire and surroundinginsulation.

Each time the receiver is turned on, the vacuum tubes receive a similarthermal shock, which materially shortens the life of the vacuum tubesinvolved.

A second effect, which occurs in the power supply system generally inuse for television receivers and which causes a further shortening ofthe life expectancy of the receiving tubes, is the application of thedirect current anode potential, which is used to control the emittedelectrons, before the cathode has reached a temperature at whichsuiiicient electron emission can occur. The result of this is to destroythe coating on the cathode. ln some cases material from the cathodemigrates to other elements producing high secondary emission ratios andmaterially affecting the vacuum tube characteristics.

During the vacuum tube warm up period the current drawn from the directcurrent portion of the receiver power supply is considerably reduced dueto the limited emission in the vacuum tubes. The decreased power supplydrain causes the voltage of the supply to increase which may temporarilyoverrate some of the receiver components.

These effects taken together make it desirable to provide a new type ofpower supply which can provide a controlled current to the heatersduring warm-up together with a delay in application ofV the power supplyanode voltage, to prevent damage to both vacuum tubes and associatedcircuit elements.

Accordingly, it is an object of this invention to provide a power supplyfor a television receiver which will `materially increase the life ofthe receiver and minimize vacuum tubes failures.

Another object of this invention is to provide a power supply whichsupplies a tube heater voltage gradually increasing as the tubes warm upto minimize the heater thermal shock.

A further object of this invention is to provide a delayed applicationof the circuit voltage for the television receiver thereby eliminatinginadvertent overloading of the circuit components and providing anextended life for the vacuum tubes.

A feature of this invention is the inclusion of a resistor element inseries with the vacuum tube heater supply having a negative temperaturecoefficient and effectively reducing the current surge associated withthe positive temperature coefficient of the vacuum tubes.

Another feature is the provision of a thermally activated time delayunit having a positive snap action to prevent contact arcing fordelaying the connection of the direct current potential of the powersupply to the television receiver circuits.

A further feature of this invention is the inclusion of a single unithaving a thermal heater element having a negative temperature coeicientand used to control the current surge in the vacuum tube heater circuitscombined with a thermally operated time delay switch responsive to theheater element for delayed application of the direct current potentialto the vacuum tubes and associated circuits.

Yet another feature of this invention is the inclusion' of a thermalresistor having a negative temperature coefficient and used to controlthe current surge in the vacuum tube heater circuits combined with athermally actuated time delay switch having a positive thermallycontrolled switching action, with the resistor heating element mounteddirectly onto a heat dissipating element to provide a stabilizedoperating temperature for the resistor, and the heat sensitive delayswitch mounted directly onto the heat dissipating element.

Further objects and features of this invention will be apparent from aconsideration of the following description and the accompanying drawingsin which:

Fig. 1 is a block diagram of a television receiver having a transformertype power supply and incorporating the thermal voltage delay system ofthis invention;

Fig. 2 is a graph showing the temperature drop of the resistance unitwith temperature and time, and the closing of the thermal time delayswitch at a given temperature and time;

Fig. 3 is a graph showing the heater voltage as a function to time;

Fig. 4 is a block diagram of a television receiver incorporating thisinvention which uses series filament connection and a voltage doublercircuit directly connected to the power line for obtaining the necessarydirect current voltages for operating the receiver;

Fig. 5 is a graph showing the voltage drop across the thermal resistanceand the series string heater voltage as a function of time;

Fig. 6 is an alternate connection for the power supply delay switchconnection in the voltage doubler type series filament receiver;

Fig. 7 shows the construction of the thermal resistor heat radiatingelements and the thermally activated switch of the invention;

Fig. 8 shows an alternate form of the invention.

The improved power supply uses a thermal resistor which is connected inseries between the power line and the TV power supply. However, sincethe thermal resistor has a negative temperature coefficient, at themoment the television receiver is turned on, and for a period of timefollowing, the temperature of the thermal 65 remains essentially zero.

i 3 heating element is near room temperature. At this temperature itsresistance is relatively high and at the same time the Vacuum tubeheater element resistancesV are lower, due to their positive temperaturecoefficient Vand the fact that the receiver has just been actuated. Theresult is that a large partv of the power line voltage appears acrossthe thermal heating resistor thereby reducing the undesired initialcurrent surge in the Vvacuum tube heating elements. At the same time therapid heating lamentary type rectifier used in the power supply isheated supplying full direct current voltage for operating the receivershortly after the receiver is turned on, `and before theheater typetubes have reached operating temperature. This D. C. voltage may exceedrated voltage for the tubes and circuit elements. A time delay switch isattached to the thermal resistor and maintains the direct currentvoltage disconnected from the receiver until the temperature of both thevacum tubes and the thermal resistor reach a specified value. When thistemperature is reached the thermal switch, which has a positive snapaction at a fixed temperature, moves rapidly to the closed positionconnecting the power supply voltage to the receiver `circuits andactivating the receiver.

Referring now to Fig. l the operation of a television receiver will bebriefly described. The television signal is received by antenna 11 whichis connected to the radio frequency selecting circuits and intermediateampliiication circuits included in section 12. This section may includethe vacuum tubes 13, 14, 15 and 16 each having a heater, 17, 18, 19 and20 respectively, connected in parallel to lead 21. The video signaldetected in the I. F. amplifier is conducted by lead 24 to the videoamplifier 25 where it is amplified by a vacuum tube 22 having a heater23. The amplified video signal is applied to control element 27 of thecathode ray display device 28 by lead 26. The cathode ray display devicealso has a heating element connected in parallel with the videoamplifier heater circuit 23 by lead 30. Lead 24 may also connect thesound signal to the sound detector amplifier and output circuits shownin 31 having vacuum tubes 32, 33 and 34 with heating elements 35, 36 and37 which are parallel connected and supplied with current through lead40. The output of the sound power amplifier 34 is' `connected toloudspeaker 39 through lead 38. Y A

Lead 24 may also supply synchronizing signals to the sync and deflectioncircuits 41 having Vacuum tubes 42, 43 and 44 with heater elements 45,46 and 47 connected in parallel on lead 48. The leads 48, 30, 40 and 21are connected in parallel thereby connecting all vacuum tube heaterelements in parallel on lead 49 which is connected to a source ofvoltage provided by winding 50 of transformer 51. The filament windingfor power rectifier 60 having a directly heated cathode 61 also islocated on transformer 51.

Windings 53 and 54 supply voltages to operate rectifier 60 as a fullwave rectifier providing a unidirectional voltage to operate the vacuumtube circuits of the receiver. The rectifier 60 output voltage isfiltered by capacitors 62 and 63 and choke 64 and applied to the spacecurrent electrodes of the vacuum tubes on lead 65. Winding 55 is theprimary winding of power transformer 51, which supplies magnetic energyfor Operating all heater and power supply windings.l

Primary winding 55 is connected to a source of A. C. power 66 throughthe main on-oif switch 59 and thermal dreppingresistor 56. Before switch59 is closed thermal resistor 56 is at normal room temperature andcontacts 57 and 53 thereof are open, keeping the center tap 67 oftransformer 51 disconnected from ground and opening vthe rectifiercircuit path so that the voltage on lead When switch V59 is closed thevacuum tube heaters having low resi-stance tend to draw a heavy currentwhich must be supplied to primary winding 55 through thermal resistor56. Since thermal resistor 56 initially has a high value of resistance alarge part of the power source voltage appears across resistor 56thereby minimizing the voltage available for the transformer winding 55and limiting the heater current surge through winding 50 and lead 49.

Referring now to Fig. 2 it can be seen in curve B, showing the variationof the resistance component with time, that after the set has ybeenturned on the resistance value of the thermal heater drops rapidlyreducing the ratio of' the voltage across resistor 56 to the Voltageacross' the transformer and providing a heater voltage as shown in Fig.3. At the same time, the temperature of resistor 56 is rising as shownin A of Fig. 2 until the temperature is reached at which contacts 53 and57 close and power' is applied activating the receiver.

When the switch is closed the effect of the thermal resistor and delay`switch is to prevent the initial heater current surge therebypreventing damage to the cathode ray tube heaters and also preventingapplication of the vacuum tube plate voltage until the heaters reachoperating temperature. i

Referring now to Fig. 4, this shows a television receiver having avoltage doubler type power supply circuit and a series filament heaterstring. The main sections of the receiver operate in the same manner asin Fig. l and will not be described again in detail. y

The heater circuits for all vacuum tubes contained in these circuits areconnected in series to ground by leads 102 to 113 inclusive and theheater element 17 of tube 13 is directly connected to thermal resistor56.V When power switch 59 is closed the voltage is applied to the seriesheater string in series with the thermal resistor 56. Referring now toFig. 5: when switch 59 is closed, the heater voltage shown in curve D islow limiting the current in the heaters while the voltage across thethermal resistor shown in curve C is high. The voltage across resistorS6 drops rapidly as shown by curve after the switch 59 has been closedand the voltage on the heaters approaches line voltage as shown in curveD. The direct current voltage for the receiver is supplied by a fullWave voltage doubler circuit including diode rectiers 114 and 115 whichmay be of any convenient type such as selenium, germanium orsilicon. Theoutput voltage of the voltage doubler circuit is filtered by capacitors62 and 63 together with choke 64. When switch 59 is closed contacts 57and 5S are initially open preventing the development of direct currentoperating voltage on lead 65. When the voltage on the series heaterstring rises to a predetermined value, the temperature of thermalresistor 56 is suicient to close contact 57 and 58 supplying voltage tolead 65 and activating the receiver. Fig. 6 shows an alternateconnection for the thermal time delay switch in the voltage doublersupply in which power is applied to the voltage doubler when switch 59is closed but connection of the power supply output to the receivercircuits is delayed by the thermal time delay switch.

Referring now to Fig. 7, in this ligure a mechanical arrangement of thethermal resistor and time delay switch isv shown. This unit may be usedin the system shown in Figs. 1, 4 and 6. The cylindrical thermalresistor element 201 has a conductive surface 202 to which is solderedat point 203 a terminal lead 204 for resistor element 201. The element201 is mechanically attached to a heat radiating plate 206 having anelectrically conductive surface which forms a second terminal of theresistor element 201. The heat conductive plate 206 Y serves the doublepurpose of a heat stabilizing sink for resistor 201V as Well asVelectrical terminal. Also attached to plate 206 and placed directlyadjacent to thermal resistor 201 is the time delay switch 207 held inplace by spring clip 211 which engages ange 210 on switch 207. Terminals208 and 209 of the switch 207 are connected tothe time delay Vcontacts57'and 58 (Fig. 6). The heat radiating plate 206 a-lso forms a means ofmounting the unit on Bakelite terminal strip 213, to which it is affixedby rivets 212. The Bakelite strip may also ymount additional terminalpoints and may be mounted to the chassis by means of bracket 215 andrivets 214.

Referring now to Fig. 8, in this figure the voltage doubler output lead65 is connected to the vacuum tube circuit as shown in Fig. 4, the lead101 may 'be the series heater string also shown in Fig. 4. The thermalyresistor 56 has connected to the bi-metal switch a second contact 58awhich provides a short circuit path around heater element 56. When theswitch 59 is closed current ows through lead 101 to the Vacuum tubeheaters and also through heater element 56 since contacts 57, 58 and 58uare open during the warm-up period of heater element 56. When bi-metalelement 57 reaches the correct operating temperature the contacts 57, 58and 58a close. This closing action may be made rapid so that the bimetalelement changes abruptly from the open to closed position to preventcontact arcing. When contacts 57, 58 and 58a close the heater elementl56 is shortened and the vacuum tubes are operated at nearly full powerline voltage. When the contact 58a shorts the heater element 56,suflicient heat is developed by current flowing through the bi-metalelement to maintain it in a closed position. When the contacts 58 closewhich may occur after the closing of 57 and 58a, the doubler circuit isactivated operating the receiver.

By using the eXtra contact 58a, heater `elementh56 no longer needs tohave a negative temperature coetiicient and therefore can be formed witha simple wire wound resistor element.

When the device of Fig. 8 is used with a power transformer circuit ofFig. l it is necessary to insulate the direct current control contactsfrom the bi-metal arm.

By using the thermal time delay device a considerable increase in tubelife occurs since the effects of heater current surge are suppressed aswell as the destructive effects of the application of direct currentplate voltage when the heater has not reached operating temperature.

I claim:

l. In a power supply system lfor operating equipment containing at leastone thermionic vacuum tube having electrodes defining a space currentpath and a cathode heater element, and which power supply systemincludes direct current voltage providing means for the electrodes andheating current providing means for the heater element, the combinationincluding, a control unit having therewith resistor means, a thermallyactivated switch, and means for mounting said resistor means and saidswitch in said control unit to provvide heat transfer therebetweenwhereby said switch is activated by heat from said resistor means, afirst circuit including said resistor means in said control unitconnecting the cathode heater element to the heating current providingmeans, said resistor means initially presenting a substantial impedanceto limit current flow to the cathode heater element and such impedancebeing reduced in response to heat produced by current flow through saidresistor means, said thermally activated switch in said control unitincluding circuit changing means having a lirst unactivated position anda second position assumed thereby in response to heat from said resistormeans, and a second circuit including said circuit changing means forapplying the direct current voltage to the electrodes of the tube, saidcircuit being open when said circuit changing means is in said firstposition so that the electrodes of the tubes are unergized and beingcompleted when said circuit changing means assumes said second positionto thereby apply the direct current voltage to the vacuum tubeelectrodes to energize the same, said circuit changing means assumingsaid second position after said resistor means reaches a predeterminedtemperature whereby the energization of the electrodes is delayed forthe period required for said resistor means to reach the predeterminedtemperature.

2. In a power supply for operating equipment containing therrnionicvacuum tubes having indirectly heated cathodes, heater elements for saidcathodes having a positive value of resistive temperature coeflicient,and anodes for drawing electrons from said cathodes, and which powersupply includes a rectifier circuit for providing a direct currentvoltage and a source of heating current for said heater elements, thecombination including a circuit for connecting the heater elements tothe source of heater current including a resistive heating elementhaving a negative value of resistive temperature coeficient, saidresistive heating element `being connected to minimize current changesin said vacuum tube heater elements resulting from said positiveresistive temperature coefficient thereof and a thermally activatedswitch having contacts normally open and closed in response to heat fromsaid resistive heating element, said switch contacts being connected inthe rectifier circuit for controlling the application of the `directcurrent voltage of the power supply to the anodes of the tubes, saidswitch operating to prevent application of the direct current voltage tothe vacuum tube anodes until said resistive heating element reaches apredetermined temperature and completing the rectifier circuit to applythe direct current voltage to the tube anodes when said resistiveheating element reaches the predetermined temperature.

3. In electronic apparatus containing thermionic vacuum tubes havingindirectly heated cathodes, and heater elements for said cathodes havinga positive value of resistive temperature coetiicient, a power supplyfor providing direct current voltage for the tubes including incombination, a transformer having primary and secondary windingsproviding voltage step-up, a full wave rectilier circuit connected tosaid secondary winding, means for providing heating current for saidheater elements including an additional secondary winding on saidtransformer, and means for controlling the application of the directcurrent voltage and the heating current to the tubes including, aresistive heating element connected in series with said transformerprimary winding, said resistive heating element having a negative valueof resistive temperature coeiiicient tending to minimize current changesin said vacuum tube heater elements resulting from said positivetemperature coeicient thereof, a thermally activated switch responsiveto heat from said resistive heating element and connected in said fullwave rectifier circuit, said switch delaying the application of saiddirect current voltage to said vacuum tubes until said resistive heatingelement reaches a predetermined temperature and completing the rectifiercircuit to apply the direct current voltage to said vacuum tubes whensaid resistive heating element reaches the predetermined temperature.

4. In electronic apparatus including thermionic vacuum tubes havingindirectly heated cathodes, and heater elements for said cathodes havinga positive value of resistive temperature coeflicient, a power supplysystem including in combination, means providing direct current voltagefor the tubes including at least one semiconductor type rectifier forconnection to a source of alternating current voltage, means forconnection to the alternating current source for applying currenttherefrom as a source of power for heating said vacuum tube heaterelements, and means for controlling the .application of the directcurrent voltage and the heating current to the tubes including, aresistive heating element connected in series with said heater elementsfor said vacuum tubes, said heating element having a negative value ofresistive temperature coeliicient tending to minimize current changes insaid vacuum tube heater elements resulting from said positive resistivetemperature coetiicient, a thermally activated switch responsive to saidresistive heating element and having normally open contacts connected insaid rectifier circuit to prevent application of said direct, currentvoltage to said vacuum tube circuits, said switch contacts closing inresponse to heat from saidheating element after said resistive heatingelement reaches a predetermined temperature. Y Y

5. In electronic apparatus containing .thermionic vacuum vtubes havingindirectly heated cathodes, -and electrical heaters for said cathodes, apower supply including first means providing a direct current voltagefor the tubes and second means for providing heating current for thetube heaters, the means for controlling the application of the directcurrent voltage and the heating current to the tubes including incombination, a first circuit portion including a resistance heatingelement connecting the tube heaters to the second power supply' means,said resistive heating element having a negative value of resistiveternperature coefficient for initially reducing the current flow to thevacuum tube heaters, a heat radiating element connected to saidresistive heating element and supporting the same, a thermally activatedswitch supported adjacent said heat radiating element and responsive tothe heat therefrom, andra second circuit portion including said switchfor applying the direct current voltage to the tubes, with said switchoperating to delay the application of the direct current voltage to saidvacuum tubes untilrsaid resistive heating element is heated to apredetermined temperature to activate said switch and apply the directcurrent voltage to said vacuum tubes.

6. In a power supply for operating equipment containing thermionicvacuum tubes having cathodes including heater elements, and anodes fordrawing electrons from said cathodes, and which power supply includesrectifier means for providing a direct current voltage and a source ofheating current for said heater elements, the combination including, acircuit for connecting the heater elements to the source of heatercurrent including a resistive heating element, said resistive heatingelement being connected to initially reduce current flow to said vacuumtube heater elements, a thermally activated switch having a first pairof contacts normally open and closed in response to heat from saidresistive heating element, va first circuit portion including saidcontacts for applying the direct current Voltage of the power supply tothe anodes of the tubes, said switch operating to apply the directcurrent voltage to the vacuum tube anodes after said resistive heatingelement reaches a predetermined.V Y temperature, said switch having asecond pair of contacts normally open and closed in response to heatfrom said resistive heating element, and a second circuit portionincluding said second pair ofcontacts for bridging across said resistiveheating element after said resistive heating elementrhas reached apredetermined temperature, so that the effect of said resistive heatingelement to reduce the current flow to said vacuum tube heater elementsis lessened.

7. In a power supply for operating equipment containing therrnionicvacuum tubes having indirectly heated cathodes, heater elements for saidcathodes, and anodes for drawing electrons from said cathodes, and whichpower supply includes rectifier means for providing a direct currentvoltage and a source of heating current for said heater elements, thecombination including a first circuit portion for connecting said heaterelements to said source of heater current including a resistive heatingelement, said resistive heating element being connected to initiallyreduce current ow to said vacuum tube heater elements, a bi-metal switchelement having a first pair of contacts normally open and closed inresponse to heat from said resistive heating element, a second circuitportion including said contacts for applying the direct current voltageof the power supply to the anodes of the tubes, said switch operating toapply the direct current voltage to the vacuum tube anodes after saidresistive heating element has reached a predetermined temperature,

f Y s Y Y said switch having a second pair of contacts normallyYopen'and closed in response to heat from said resistive heating element,and a third circuit portion including said second pair of contacts andsaid bi-metal switch element for bridging across said resistive heatingelement 'after said resistive heating element has reached apredetermined temperature, said third circuit portion having a currentflowing therein to increase the current flow to said vacuum tube heaterelements, said current flowing in said bi-metal switch element operatingto heat the ,same to maintain said first andV second pairs of contactsin a closed position.

8. In a power supply system for operating equipment containing at leastone thermionic vacuum tube having electrodes defining a space currentpath and a cathode heater element, with the cathode heater elementhaving a positive value of resistive-temperature-coefcient whereby thereis a current surge when the heater element is energized, and which powersupply system includes direct current voltage providing means for theelectrodes and heating current providing means for the heater element,the combination including, a resistive heating unit in a first circuitconnecting the cathode'heater element to the heating current providingmeans, said resistive heating unit having a negativeV value ofresistive-temperaturecoefiicient so that said resistive heating unitinitiallyv presents a substantial impedance to limit the current surgeto the cathode heater element and is responsive to heat produced bycurrent iiowing through said resistive heating unit whereby theimpedance to current flow is reducedQas the temperature rises, athermally activated switch positioned relative to said resistive heatingunit so as to `be responsive to heat therefrom, means for Y mountingsaid switch and said resistive heating unit to provide the positioningwhich accomplishes heat transfer therebetween, said switch includingcircuit changing means having a first unactivated position and having asecond position assumed thereby'in response to heat from said resistiveheating unit, and a second circuit including said circuit changing meansfor applying-,the direct current voltage to the electrodes of the tube,said second circuit being open when said circuit changing means is insaid first unactivatedposition so that the electrodes of the tube areunenergized and said circuit `being completed when said circuit changingmeans assumes said second position to thereby apply the direct currentvoltage tothe vacuum tube electrodes and energizerthe same,

said circuit changing means of said thermally activated ergized, a powersupply for providing direct current voltage and heater current for thetube including in combination, a transformer having primary and firstland second secondary windings, means for connecting said primarywinding to an alternating current supply, a rectifier circuit connectedto said first secondary vwinding for providing the direct currentvoltage, heating current providing means for said heater elementincluding said 'second secondary winding, and control meanstforcontrolling the application of the'direct current voltage andthe heatingcurrent to the tube, said control means including therewith a resistiveunit and a thermallyactivated switch, means connecting said resistiveunit inrseriesrwith said transformer primary winding, said resistiveunit ,initially presenting a substantial resistance to reduce currentfow to said transformer and thereby reduce the current surge 9 in saidheater element, said resistive unit being responsive to heat produced bycurrent flow therein to decrease such resistance and thereby tend tominimize changes in current flow in said heater element resulting fromsaid positive temperature coeiiicient thereof, said thermally activatedswitch including normally open contacts connected in said rectifiercircuit, said switch ybeing responsive to heat produced by current flowin said control means to close said contacts when said switch reaches apredetermined temperature and thereby delay the application of saiddirect current voltage to said electrodes of the vacuum tube.

10. In electronic apparatus including at least one thermionic vacuumtube having a cathode heater element with a positive value ofresistive-temperature-coeicient whereby there is a current surge whenthe heater element is energized, a power supply system including incombination, current supply means for connection to an alternatingcurrent source, means providing direct current voltage for the tubeincluding at least one semi-conductor type rectier connected to saidcurrent supply means, means for providing current for the vacuum tubeheater element from said current supply means, and means for controllingthe application of the direct current voltage and the heating current tothe tube including control means having therewith a resistive unit and athermally activated switch, means connecting said resistive unit inseries with the heater element of said vacuum tube, said resistive unitinitially presenting a substantial resistance to reduce the currentsurge in the heater element and being responsive to current owtherethrough to reduce such resistance and thereby tend to minimizecurrent changes in said vacuum tube heater element resulting from saidpositive resistive temperature coefficient, said thermally activatedswitch having normally open contacts connecting said rectifier to thetube to delay the application of said direct current voltage to thevacuum tube, said switch being responsive to heat produced by currentflow in said control means to close said contacts when said thermallyactivated switch reaches a predetermined temperature to thereby applysaid direct current voltage to said vacuum tube.

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